Voltage controlled resistors, having a resistance which varies with an applied voltage, have use in a wide variety of applications, including, for example, tuning circuits. Metal oxide semiconductor (MOS) transistors may function as voltage controlled resistors if operated in the ohmic region, with the gate-source voltage controlling the resistance.
Within the signal path of a signal processing circuit, however, the resistance of an MOS transistor employed to provide voltage controlled resistance also changes with the source-drain voltage. If the gate voltage is held constant, the resistance of the transistor changes with the source voltage, introducing high non-linearity in the resistive behavior of the transistor. This non-linearity becomes higher as the overdrive voltage applied to the transistor—the gate-source voltage minus the threshold voltage (Vgs−Vt)—decreases. As the voltage supply becomes lower, providing a good overdrive of the transistor becomes increasingly difficult, particularly if the source (assuming an n-channel transistor) cannot be connected to ground.
There is, therefore, a need in the art for a voltage controlled resistor having a high linearity of resistance per unit change in applied voltage.